1. Technical Field
The present disclosure relates to a startup circuitry connected to a main circuit which has at least an output terminal connected to its feedback terminal by a feedback loop. The disclosure also relates to a method for providing a startup correction to a main circuit connected to a startup circuitry. The disclosure particularly, but not exclusively, relates to a startup circuitry for analog circuits, in particular of the low power type and the following description is made with reference to this field of application for convenience of explanation only.
2. Description of the Related Art
As it is well known, self biased circuits, in particular analog ones, need to have a so called startup circuitry able to fast reestablishing a working or operational state when the circuit are connected to a power supply or, in a general manner, when they are enabled to work.
In particular, circuits with an internal feedback loop, like bandgap reference circuits, DVgs/R reference circuits or current source circuits, which are used widely in nowadays analog integrated circuits, usually have more than one single desired operational status or point.
Therefore, in this field there is the need of ensuring that a circuit of this kind reaches a right DC operational point, in a safely and reasonably timely manner. This need is satisfied, as already indicated above, by adding a suitable startup circuitry to main circuit.
A startup circuitry is generally divided in two portions, namely a first or sensing portion that detects whether a main circuit connected to the startup circuit is in a desired operational point (OP) and suitably drives a second or active portion.
In particular, the active portion of the startup circuitry is configured to force the main circuit to a desired OP, while both the sensing and the active portions should not influence the main circuit when settled in the desired OP.
A startup circuit of a known type is shown for example in FIGS. 1A and 1B and described in the U.S. Pat. No. 5,867,013 issued on Feb. 2, 1999 in the name of CYPRESS SEMICONDUCTOR CORP. In particular, according to this known solution, the sensing of the OP of a band-gap reference circuit 30 is based on the voltage value Vgs of the NMOS transistor 50 which senses the output band-gap voltage Vout at the node 46. The signal so sensed is shaped and processed by the inverter 56 of the startup circuit 44. Moreover, the output signal from the inverter 56 is provided to an active portion of the startup circuitry 44 essentially comprising the PMOS transistor 64. This active portion forces, in case of a bad operational state of the band-gap reference circuit 30, a positive voltage value to the node 48.
Another known startup circuitry is shown in FIG. 2. In this case, a branch current rather than a node voltage is checked by the sensing part of the startup circuit to recognize the state of the main circuit, in particular a band-gap circuit. A sensing transistor M1 copies a current signal from the main circuit and compares it with the current supplied by the resistor R3. The active portion of the startup circuit essentially comprises the transistor M2. In case of a wrong OP (when there is no current through the main circuit branches) the transistor M2 starts to sink current and forces the band-gap circuit to the right OP.
The many different startup circuitries, which are known and widely used in analog circuit design, could be considered as alternative embodiments or improvements of the above described circuitries. Some of them are described for instance in the U.S. Pat. Nos. 6,160,392, 6,084,388, 6,222,399, 6,784,652 and 6,057,721.